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Prediction of Gravity Anomaly from Calculated Densities of Rocks

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Prediction of Gravity Anomaly from Calculated Densities of Rocks Available online a t www.pelagiaresearchlibrary.com Pelagia Research Library Advances in Applied Science Research, 2012, 3 (4):2059-2068 ISSN: 0976-8610 CODEN (USA): AASRFC Prediction of gravity anomaly from calculated densities of rocks Omosanya K. O.1,2* , Mosuro G. O1, Laniyan T. A1 and Ogunleye D. 1 1Department of Earth Sciences, Olabisi Onabanjo University, Ago-Iwoye, Nigeria 2Cardiff University, School of Ocean, Earth and Planetary Science, United Kingdom _____________________________________________________________________________________________ ABSTRACT Density and density contrasts of rocks controls gravity anomalies and is an important physical property of geologic material which aids in the identification of rocks, estimation of ore abundances, and assessing rock conditions. The study was carried out with the aim of modeling gravity anomaly from calculated densities. Dry density of twenty five (25) rock samples collected randomly during the geological mapping exercise were analyzed in the laboratory using traditional dry and wet method. The particle density was estimated using the relationship (m2 - m1) / (m4-m1) - (m3-m2). Rocks identified in the area include Granite (av. ρ ~2.72 gm -3), Biotite granite (av. ρ ~2.7213gm -3), and Pegmatite (av. ρ ~ 2.7125 gm -3). Anomaly types described along selected gravity profiles include S, M, Concave up, UL, Zig-Zag Flat top and Stair case anomalies. The 3D modeled gravity anomaly map of the upper part of the study area shows that peaks in the map correspond to the location of the pegmatite outcrop. Along the profiles the pegmatite outcrop are shown as concave up anomalies. Gravity anomalies in the study area are associated with mineralization and ambiguity in the anomalies calculated result from variation in the porosity of the different rock samples. Keywords: Density, Gravity, Anomaly, Pegmatite, Mineralization _____________________________________________________________________________________________ INTRODUCTION Gravity anomalies result from the difference in density, or density contrast, between a body of rock and its surroundings. Gravity data are used to constrain subsurface density variations associated with tectonics and mineralization. The force of gravity at the earth surface is affected by lateral changes of density due to the total mass of the earth. Two components of gravity forces are measured at the earth’s surface, a general and relatively uniform component due to the total earth and a smaller size which varies due to lateral density changes. The latter generally referred to as gravity anomaly is the difference between the observed earth’s gravity and a value predicted from a model (Fricke & Schön 1999, Rider 1996, Serra 1984). The density of a rock is the ratio of its mass per volume. For specific rock types, density has less percent variability especially at specific places. Due to the low porosity and permeability of igneous and metamorphic rocks, the densities are considered close approximate of the field values (Subrahmanyam and Verma, 1981). Unconsolidated materials such as alluvium and stream channels materials may have significant variation in density, due to compaction and cementation. The influence of porosity on density in hard rocks is usually less than one per cent (Henkel, 1976); low density values from igneous rocks may be due to the weathering (Ranganai, 1995) of such rocks. Direct knowledge of density has been used to predict gravity anomalies without any need for time, elevation, instrument drift, and other gravity corrections especially in basement complex environment. When accurately calculated, densities of rock is essential in petrological and geological studies and more so for a meaningful structural interpretation of gravity anomalies (Ajakaiye & Burke 1972, Ajakaiye & Sweeney, 1974). 2059 Pelagia Research Library Omosanya K. O. et al Adv. Appl. Sci. Res., 2012, 3(4):2059-2068 _____________________________________________________________________________ Fig.1: Index map of the study area, accessibility to outcrop is enhanced by major roads and footpaths. Climate is tropical with a sub-dendrite like drainage pattern. The thrust of this research is to calculate densities of rock samples in the laboratory with a view to predicting gravity anomalies and elucidating the geometry of the pegmatite intrusion in the study area especially as it relates to its representation on the geological map as tabular or elliptical bodies. This work starts by discussing the geology of the area based on the result of the geological mapping exercise, the methods used for the study and the implications of the result. Local Setting The study area, is located with longitude E 3 0 56 I - E 4 0 00 I and latitude N 6 054 I - N 6 0 58 I covering settlements such as Awa, Ilaporu, Oru, Ibakan-Isale , Talafanmu, Iganran, Iganran-Isale, Aparaki, Imope, and Odo-alamo respectively. The study area fall within the tropical rain forest region of Nigeria with most of the parts covered by vegetation. Temperature ranges from 25 oC to 32 oC (Akintola, 1986) alternating wet and dry seasons with maximum rainfall of ~1500mm. The topography is wavy or undulating; with the drainage systems flowing NE-SW direction in sub-dendrite like manner. MATERIALS AND METHODS Initial field mapping exercise involved the observation of rocks at outcrop locations, determination of positions using the Garmin GPS in longlat coordinate system, description of mineralogical composition, texture, degree of weathering of rock and collection of twenty four (24) fresh samples for further laboratory studies. 2060 Pelagia Research Library Omosanya K. O. et al Adv. Appl. Sci. Res., 2012, 3(4):2059-2068 _____________________________________________________________________________ In the laboratory, the samples were heated in an oven kept at a temperature 100 0c for 24 hours under reduced pressure in order to remove all water content and were weighed in air (Mass of Pycnometer + rock particles + water (m 3) and quickly in water(Mass of Pycnometer + rock particles (m 2) using a Mettler balance. All samples were then saturated in water at reduced pressure for 24 hours, weighed again in water (Mass of Pycnometer full of water (m 4) and in air (Mass of Pycnometer (m 1). From the result obtained, the particle density was estimated from the relationship (m 2 - m1) / (m 4-m1) - (m 3-m2). For the density measurements the following parameters were determined for each of the samples: pore volume, bulk volume, or grain volume and weight. Statistical analysis of the data included determination of mean, mode, median and standard deviation. The 3D gravity anomaly map was computed by plotting estimated density contrast against spatial position on Surfer 8. Longitude and latitudes coordinates were converted into XY (UTM), loaded onto a spreadsheet and the map was gridded using the density values as the z-factor, elevation map of the area was also plotted. The anomaly map was divided into two based on the position of the outcrops in order to avoid overestimation of the density contrasts over locations where rocks are not outcropping. Cross sections were also drawn along selected profiles to further elucidate the gravity anomaly. In estimating the gravity anomaly, a contrast of <0.2 g/cm 3 was calculated between the granitic rocks and the continental crust which coincides with the temperature less than the granite solidus at depth of <20km (Rast, 1970, Ajakaiye, 1974). RESULTS AND DISCUSSION Geology of the study area The general geology of Nigeria have been previously studied by Rahaman (1971), Oyawoye (1972), Cooray (1972), Elueze (1981), Caby et al (1981), Dada et. al. (1995), Dada (1998), and more recently Omada & Olorunfemi, 2012, Oden & Igonor, 2012, Ekwere & Edet, 2012, Bassey, 2012. The crystalline rocks in Nigeria are distributed in a circular area in the North central, a triangular area in the West which runs into the Benin Republic and a rectangular area broken into three parts by sedimentary rocks on the Eastern border of Nigeria with Cameroun Republic. The Precambrian rocks of Nigeria, collectively known as the basement complex occupy nearly half the total area of the country, and the other half is covered by the Cretaceous and younger sedimentary rocks The rocks of the study area include Biotite Granite, Granite and Pegmatite (fig.2). There is preponderance of Biotite Granite from North to South, leucocratic rocks characterized by Felsic minerals that generally trend NNW-SSW. The Granites are emplaced on the Eastern and NW margin of the study area where they are flanked by Pegmatite intrusions. The rocks are thought to belong to the older Granite suites (Falconer, 1911) of the Precambrian Basement complex of Nigeria. This rock suite are generally calc alkaline, orogenic rocks of Pan African age (+500ma) and include such rocks as granite, Granodiorite, Adamellite, Quartz, Monzonite, Syenite, and pegmatites. The granites of the study area are main phase granite bearing dominant minerals such as hornblende and biotite. Other minerals identified in the granites include quartz, k-feldspar and muscovite in subordinate amount to the biotite and hornblende; the rocks generally are phaneritic in texture. The pegmatite occurs both as outcrop and intrusions in other rock types. The minerals are of large grain and coarse texture with; mineralogical compositions include muscovite, tourmaline, quartz and sometimes with dark minerals such as biotite. Densities The highest density values was recorded in the biotite granite
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